A recent report of the Committee for the Review of Pulmonary Devices Studies has indicated that one of the top priority areas in the study of pulmonary disease is instrumentation development. The ultrasonic ventilometer presented here is intended to alleviate many of the measurement problems associated with traditional airflow metering methods, and, as a technique, has been specifically recommended by the National Academy of Engineering Subcommittee on Technology and Systems Transfer in its report of 5 August 1971. Several techniques utilizing transit time, phase, or frequency measurements will be examined (and extended, where necessary) both analytically and experimentally for their utility in the development of an ultrasonic gas flow meter suitable for clinical and research applications. Emphasis is on first creating an accurate theoretical model of the measurement system, including as many of the known noise sources as possible (e.g., variations in gas properties -- temperature, pressure, composition -- acoustic and electrical channel noise, and dimensional stability) and examining each of the common measurement schemes (pulse- transit time, sing-around, phase shift) for their sensitivities to these corrupting influences. Experimental work consists of converting the best of the theoretically feasible systems to hardware and will involve specific work in four areas: (1) Transducer development for desired bandwidth and beam shapes; (2) Signal processing algorithm development; (3) Examination of a new phase-shift technique currently under development at the University of Virginia; (4) Test and evaluation of prototype devices. The efforts described herein should make significant contributions to the general field of theoretical ultrasonic aneneometry as well as to the applications which are restricted to the medical context.